For months I've been getting the check engine light appearing on my dash. It's code P0156 which is 02 sensor malfunction. I posted about the problem before but the light went out and the car has continued to run fine. Occasionally the light will come on but it doesn't last more than a day and it's gone (it can go weeks before it reappears).

So yesterday another warning light appears and it's claiming my gas cap is not tight. I go and have a look and everything is fine. After the car had been sitting for about 8 hours and I went to start it up, the light had now disappeared.

I used my code reader and the only code that was showing up was the one for the faulty gas cap. When I checked the codes again later that day, it came back with ZERO codes!. Even the 02 sensor code which has been there for months had disappeared. So it was almost like nothing was wrong with the car.

Today the code for the 02 sensor has appeared again but the gas cap light and code has not. I've purchased a new cap and will install tomorrow to make sure it's ok.

I should note that the car went in for an emissions test 3 weeks ago and passed with flying colours.

Codes can be intermittent if the problems are intermittent, so it's not unexpected. The code for your O2 sensor is probably a sign that the sensor is starting to get lazy or the heater is wearing out. You can inspect the wiring, but if it looks good I would just replace the sensor (it's the downstream sensor for the front bank). Have you tried replacing this sensor before?

Intermittent EVAP codes can also be difficult to track down. I'm guessing you had a P0455? That one can be a bad gas cap seal, so it's a good idea to replace the cap. It could also be a purge solenoid that is sticking, or possibly a bad fuel tank pressure sensor. The EVAP leak monitor only runs under specific circumstances, so if you use the car a few times after setting the code (but before the leak monitor can run again), the CEL will go out. When the leak monitor runs again, then the CEL will come back on. So in that sense it might seem like an intermittent problem, but it might not be actually.

Gas cap for the evap first, since I see Northern that means salt and possible rusty gas filler tube issues. Do a search on that if the cap doesn't fix the evap code.
I had a rusty filler tube issue on my car BTW.

Codes can be intermittent if the problems are intermittent, so it's not unexpected. The code for your O2 sensor is probably a sign that the sensor is starting to get lazy or the heater is wearing out. You can inspect the wiring, but if it looks good I would just replace the sensor (it's the downstream sensor for the front bank). Have you tried replacing this sensor before?

Intermittent EVAP codes can also be difficult to track down. I'm guessing you had a P0455? That one can be a bad gas cap seal, so it's a good idea to replace the cap. It could also be a purge solenoid that is sticking, or possibly a bad fuel tank pressure sensor. The EVAP leak monitor only runs under specific circumstances, so if you use the car a few times after setting the code (but before the leak monitor can run again), the CEL will go out. When the leak monitor runs again, then the CEL will come back on. So in that sense it might seem like an intermittent problem, but it might not be actually.

The gas cap warning light has not returned so maybe the problem is solved. Guess time will tell.

As for the 02 sensor.... I'm afriad I've never had to replace it before. I would prefer doing it myself but the diagrams that I've seen aren't very clear on where I can expect to find it. I even did a search on Youtube hoping somebody had posted a video. Even if I had a picture... it would at least point me in the right direction.

Somebody suggested running seafoam through the car. Not sure if that would solve any problems.

Bank 2 is the front (radiator side) of the engine. Just follow the exhaust and you should see the downstream sensor after the first cat. I believe the sensor will have a blue connector. I'm guessing you'll have to get to it from below.

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